A continuous challenge in semiconductor processing is to improve conductivity and performance of stacked semiconductor structures. Among the stacked semiconductor structures commonly utilized are gate stacks, wordlines, programmable-read-only-memory devices such as EPROMs and EEPROMs, and conductive interconnects. Formation of some of these prior art stacked structures is described with reference to FIGS. 1-4. FIGS. 1-2 pertain to the formation of a wordline or gate stack structure, and FIGS. 3-4 pertain to the formation of a programmable-read-only memory device.
Referring to FIG. 1, a semiconductor wafer fragment 10 is illustrated at a preliminary processing step of a prior art process for forming a wordline or gate stack. Wafer fragment 10 comprises a semiconductive material substrate 12, and field oxide regions 14 over substrate 12. A gate dielectric layer 16, generally comprising silicon dioxide, extends between field oxide regions 14. A polysilicon layer 18 and a polycide (silicide) layer 20 are formed over field oxide regions 14 and gate dielectric layer 16.
Polysilicon layer 18 typically comprises polysilicon uniformly doped with a conductivity enhancing dopant (illustrated by stippling within layer 18). Polycide layer 20 comprises a metal silicide, such as tungsten silicide, molybdenum silicide, titanium silicide or cobalt silicide. The formation of polycide layer 20 typically comprises depositing a metal over polysilicon layer 18 and reacting the metal with polysilicon layer 18 to form a metal-silicide. The reacting can comprise thermal processing of the metal layer and polysilicon layer at, for example, temperatures of from about 600° C. to about 800° C.
Referring to FIG. 2, layers 16, 18 and 20 are patterned to form a conductive stack, and specifically to form a wordline 24. Source/drain regions 25 are provided proximate wordline 24. Conductive wordline 24 comprises a transistor gate electrically connecting source/drain regions 25. The final transistor structure can be either a p-channel transistor (PMOS), or an n-channel transistor (NMOS), and can be incorporated within a CMOS construction.
The speed of devices comprising wordlines and conductive gates generally increases with increasing conductivities of the wordlines and conductive gates. Accordingly, it would be desirable to improve the conductivity of wordlines and transistor gates. A method for improving the conductivity of a doped layer is to “activate” the dopant within the layer. Although the chemistry of dopant activation is not well understood, activation is thought to occur as dopant is dispersed from grain boundaries in a polysilicon layer to bulk polysilicon away from the grain boundaries. Dopants are typically activated by thermal processing.
Alternative procedures similar to those of FIGS. 1 and 2 can be used to form a conductive polysilicon interconnect. Such interconnects can comprise a line of polycide over a polysilicon. Accordingly, such interconnects are similar to wordline 24, but lack dielectric layer 16.
The speed of devices comprising conductive interconnects can increase with increasing conductivities of the conductive interconnects. Accordingly, it would be desirable to improve the conductivity of conductive interconnects.
Referring to FIGS. 3-4, a prior art process for forming a programmable-read-only memory (PROM) device is illustrated. In the embodiment of FIGS. 3-4, similar numbering to that of the embodiment of FIGS. 1-2 is utilized, with differences indicated by the suffix “a”, or by different numbers.
Referring to FIG. 3, a wafer fragment 10a is illustrated at a preliminary step during formation of a programmable-read-only memory device. Wafer fragment 10a comprises a semiconductive material 12a over which is formed field oxide regions 14a and gate dielectric layer 16a. A first polysilicon layer 18a is formed over regions 14a and dielectric layer 16a. A second dielectric layer 26 and a second polysilicon layer 28 are formed over first polysilicon layer 18a, and a polycide layer 30 is formed over second dielectric layer 26.
Polysilicon layers 18a and 28 comprise uniformly doped polysilicon, typically comprising a dopant concentration of greater than 1×1019 ions/cm3.
Referring to FIG. 4, layers 16a, 18a, 20a, 26, 28 and 30 are patterned to form the resulting PROM device 32. Within device 32, the patterned first polysilicon layer 18a is typically referred to as a floating gate. The patterned second polysilicon layer 28 and polycide layer 30 together comprise a conductive line 33.
The speed of circuits comprising PROM devices can increase with increasing conductivities of the conductive line and floating gate. Accordingly, it would be desirable to improve the conductivities of conductive lines and floating gates.